Detailed explanation of the use and writing of callbacks in PHP

With the development of Internet technology, PHP has become one of the important languages ​​​​for website development. In PHP development, the callback function is a very important language feature and is often used. This article will introduce the use and writing of callbacks in PHP.

1. What is a callback function

The callback function refers to passing the function name as a parameter to another function in the function call. When the function is called, the function name passed in will be Used inside this function. Simply put, a callback function is a function that is passed as a parameter to another function.

2. Callback function usage scenarios

Callback functions are widely used in PHP and are mainly used in the following three aspects:

1. Event processing

In event-driven programming, it is often necessary for a function to be automatically executed when an event occurs. Such a function can be called a callback function. For example, the event binding function in jquery will only call the callback function when the event occurs.

2. Sort

In PHP, the sort function has two main parameters, which are the sorted array and a comparison function. The comparison function is a callback function that is used to define how to compare items in the array according to requirements. For example, the usort() function and uasort() function use callback functions to compare items in an array.

3. Dynamic call

When we write a general function, we often need to dynamically call different functions. At this time, we can use callback functions.

3. How to write callback functions

There are two ways to pass callback functions in PHP: anonymous function and callback function name. Below we will introduce these two writing methods respectively:

1. Anonymous function

After PHP5.3 version, support for anonymous functions has been added, and you can directly call the function Define anonymous functions. The following is a sample code for an anonymous function:

function array_sort($arr, $compare_func) {
   usort($arr, $compare_func);
   return $arr;
}

//仅使用一次的函数可以通过匿名函数来定义
$sorted_array = array_sort($arr, function($a, $b){
   return strcmp($a->name, $b->name);
});

In the above code, the anonymous function is passed directly as a parameter to the array_sort function.

2. Callback function name

The callback function name can be a string or an array.

(1) The callback function name is a string

When the callback function name is a string, the advantage is that it is simple and easy to use. The following is a sample code when the callback function is named a string:

function my_callback_function() {
    echo 'hello world!';
}

call_user_func('my_callback_function');

In the above code, we called the my_callback_function() function through the call_user_func() function.

(2) The callback function name is an array

When the callback function name is an array, and the first element is an instance of an object, the second element must be this object a public method. The following is a sample code when the callback function is named an array:

class MyClass {
    public static function myCallbackMethod() {
        echo 'Hello World!';
    }
}

call_user_func(array('MyClass', 'myCallbackMethod'));

In the above code, we call the myCallbackMethod method of the MyClass class through the call_user_func() function.

4. Notes on callback functions

1. The callback function must be a function that has been declared in PHP or an object method that can be called.
2. The callback function must be declared or defined before calling the function, otherwise an "undefined function name" error may occur.
3. When the callback function is a method that uses an object instance, the object must be instantiated before calling the function.

Summary

The callback function is a very commonly used language feature in PHP and can be used in scenarios such as event processing, sorting, and dynamic calling. Anonymous functions and callback function names are two commonly used ways of writing. Using PHP, we can define and use callback functions very conveniently.

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